Process of equalizing resistance of enlarged ends of electric heating elements



Sept. 5, 1933. c. H. BOYLES 1,925,129

PROCESS OF EQUALIZING RESISTANCE OF ENLARGED ENDS OF ELECTRIC HEATING ELEMENTS Filed June 27, 1932 INVENTOR ATTORNEYS Patented Sept. 5, 1933 PROCESS OF EQUALIZING RESISTANCE F ENLARGED ENDS OF ELECTRIC HEATING ELEMENTS Carroll H. Boyles, Milwaukee, Wis.

Application June 27, 1932. Serial No. 619,502

, 12 Claims.

In my application Serial No. 616,961, filed June 13, 1932, I have disclosed a method of making an elongated electrical heating element consisting principally of silicon carbide. In certain instances it is desirable to enlarge the ends of such elements, in order to provide a greater conductivity at the ends, so that the lead-in wires may be connected directly to the ends in service, and at the same time furnish ample current carrying capacity-to the body of the heating element.

In order to attain this desired result, it is also necessary that the unit resistance of the enlarged ends be equal to, or at least not greater than, the unit resistance of the intermediate bodyportion. In the manufacture of the element, .how-

ever, these conditions do not always occur; and the object of the present invention, therefore, is to provide means for equalizing the unit resistance of the enlarged ends to that of the intermediate body portion.

As described in the aforementioned application, the heating element is completed and rendered conductive by the passage of current through its entire length. In order to equalize the resistance of the enlarged ends, according to the present invention, these ends are given a subsequent current treatment which however must not be applied to the intermediate body portion. The

invention provides a simple and emcient means for the additional electrical treatment of the enlarged ends to the exclusion of the intermediate body portion. The method consists essentially in connecting both ends into an electrical circuit and short-circuiting the intermediate body portion by coating the same with a material which is a much better conductor than the material constituting the heating element. The conducting material is applied in the form of a liquid or fluid and is of such a character that it solidifies on heating and may be removed from the heating element when cooled. In the preferred embodiment of the method, several such elements are connected end to end or in series by a conductor such as graphite paste, and the current is passed through the whole series.

The invention is more fully disclosed with reference to the accompanying drawing, in which-- Figure 1 is an elevation of an element with enlarged ends to be treated according to this invention;

Fig. 2 is a longitudinal section of the trough furnace, showing a series of elements embedded therein; and

Fig. 3 is a corresponding transverse section.

Reference to these'views will now be made by use of like characters which are employed to designate corresponding parts throughout.

The heating element is an elongated member having an intermediate body portion 1 and enlarged end portions 2, as shown in Figure l,

whereby these end portions may ultimately have greater conductivity than the intermediate portion, so that sufficient current will be carried to the intermediate portion if the lead-in wires are attached directly to the ends. The element may be or" any desired cross sectional shape, both at the intermediate and end portions, and consists mainly of silicon carbide as disclosed in the aforementioned pending application.

The element is made operatively conductive of electrical current by the passage of current therethrough from end to end, but it does not always follow that the enlarged end portions have a unit resistance equal to 01' less than that of the intermediate portion, which is necessary in order to bring about the desired conductivity relations above mentioned.

The unit resistance of the enlarged ends is equalized to that of the intermediate portion by passing a current through the ends, exclusive of the intermediate portion, and this passage of current is in addition to the electrification described in the above mentioned application for fusing the particles of carbide together. The intermediate portion 1 is excluded from the action of a current passed through the ends 2, by coating the portion 1 with the material 3 having a much'higher conductivity than the carbide material constituting the element proper. This material is applied in liquid or plastic form, generally designated in the claims as fluid, and preferably contains carbon or a substance which forms carbon on being heated, mixed with graphite or similar material having a comparatively high electrical conductivity. The coating material must further solidify on being heated and be capable of being removed from the element when cold. Examples of suitable materials for this purpose are coal tar, molasses, dextrine compounds, sulphate compounds and colloidal graphite mixed with graphite or a similar conductor, if necessary, to produce a high conductivity when solidified by heat treatment.

A number of elements thu's coated at their respective intermediate portions 1 are connected in series by means of carbon paste 4 or similar means which holds the elements together under heat. Carbon electrodes 5 are connected to the ends of the series in like manner and are placed in a furnace 6. It is preferable to pack the elements with a non-gassing material 7 which holds the heat and has no chemical action on the elements. This material may be the packing material described in the aforementioned application, consisting of 60% silica, 30% carbon, 10% colloidal clay or Bentonite the whole being calcined to 4500" F., pulverized and screened. The lead-in wires 8 are clamped at 9 to the carbon electrodes 5, and the circuit further contains well known mechanism for controlling the voltage and current according to the resistance of the ends 2. A controlled and regulated current is passed through the ends 2 until their unit resistance is reduced to that of the intermediate portion 1; On completion of the electrical treatment, the elements are removed from the furnace and separated from one another, and the solidified coating material is scraped oil or otherwise removed.

The amount of current passed through the enlarged ends in the subsequent treatment is so calculated that the total amount of current passed therethrough shall equal the amount of current passed through the intermediate portion of the rod, per unit of cross sectional area. For exam ple, if an element having a cross sectional area of one-half square inch in the body is treated at thirty amperes for fifteen minutes and the end sections have a cross sectional area of threequarters square inch, the total amount or" current applied to the end sections is forty-five amperes for -fifteen minutes. In practice, the current applied to the end sections may slightly exceed the calculated amount.

The scope of the invention is indicated by the appended claims rather than by the particular details illustrated and described herein, as various alterations and modifications in procedure will readily suggest themselves to persons skilled in the art.

What I claim is:--

1. The method of equalizing the unit resistance of an elongated heating element having enlarged ends of different unit resistance than the intermediate portion, consisting in inserting the element in an electrical circuit, short-circuiting said intermediate portion, and passing electrical current through the circuit.

2. The method of equalizing the unit resistance of an elongated heating element having enlarged ends of different unit resistance than the intermediate portion, consisting in inserting the element in an electrical circuit, coating said intermediate portion With a material having a higher conductivity than the element material, whereby to short-circuit said portion, passing electrical current through the circuit, and removing said coating material.

3. The method of equalizing the unit resistance of an elongated heating element having enlarged ends of different unit resistance than the intermediate portion, consisting in inserting the element in an electrical circuit, coating said intermediate portion with a material having a higher conductivity than the element material and adapted to be removed when cold, whereby to short-circuit said portion, passing electrical current through the circuit, and removing said coating material.

4. The method of equalizing the unit resistance of an elongated heating element having enlarged ends of different unit resistance than the intermediate portion, consisting in inserting the element in an electrical circuit, coating said intermediate portion with a fluid material having a higher. conductivity than the element material, whereby to short-circuit said portion, passing electrical current through the circuit, and removing said coating material.

5. The method of equalizing the unit resistance of an elongated heating element having enlarged ends of different unit resistance than the intermediate portion, consisting in inserting the element in an electrical circuit, coating said intermediate portion with a fluid material having a higher conductivity than the element material and adapted to be removed when cold, whereby to short-circuit said portion, passing electrical current through the circuit, and removing said coating material.

6. The method of equalizing the unit resistance of an elongated heating element having enlarged ends of different unit resistance than the intermediate portion, consisting in inserting the element in an electrical circuit, coating said intermediate portion with a material which, on heating, is solid and contains asubstantial amount of carbon and has a higher conductivity than the element material, whereby to short-circuit said portion, passing electrical current through the circuit, and removing said coating material.

7. The method of equalizing the unit resistance of an elongated heating element having enlarged ends of difierent unit resistance than the intermediate portion, consisting in inserting the element in an electrical circuit, coating said inter mediate portion with a fluid material, which on heating, solidifies and contains a substantial, amount of carbon and has a higher conductivity than the element material, whereby to shortcircuit said portion, passing electrical current through the circuit, and removing said coating material.

8. The method of equalizing the unit resistance of an elongated heating element having enlarged ends of different unit resistance than the intermediate portion, consisting in inserting the element in an electrical circuit, short-circuiting the intermediate portion, packing the element in a heat insulating material, and passing electrical current through the circuit.

9. The method of equalizing the unit resistance of an elongated heating element having enlarged ends of difierent unit resistance than the intermediate portion, consisting in inserting the element in an electrical circuit, short-circuiting said intermediate portion, packing the element in a calcined mixture of 60% silica, 30% carbon and 10% colloidal clay, and passing electrical current through the circuit.

10. The method of equalizing the unit resistance of elongated heating elements having enlarged ends of different unit resistance than the intermediate portions, consisting in connecting a plurality of such elements in series in an electrical circuit, short-circuiting said intermediate portions, and passing electrical current through the circuit. i

11. The method of equalizing the unit resistance of elongated heatingelements having enlarged ends of difierent unit resistance than the intermediate portions, consisting in connecting a plurality of such elements in series in an electrical circuit, short-circuiting said intermediate portions, packing the elements in a heat insulating material, and passing electrical current through the circuit.

12. The method of equalizing the unit resistance 

